Thank you. Just what I was looking for.
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user16104Jun 27 '14 at 16:04

This is an extremely common class of question. In addition to the Linked questions under the original see also e.g. (11219), (11780), (30155), (33880). For clarity I would use: Cases[list, {a_, b_, ___} /; a + b < 4]. For speed choose numeric methods using e.g. UnitStep.
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Mr.Wizard♦Jun 27 '14 at 19:46

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